Heat treating process



Patented Feb. 11, 194i PATENT 2,231,010 nsa'r 'rnnarrno rnocEss DonaldA. Holt, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours &Company, Wilmington, Del., a corporation of Delaware No Drawing.Original application February 24,

1938, Serial N0. 192,396.

Divided and this application May 20, 1940, Serial No. $36,178

t Claims.

This application is a division of my copending patent application SerialNo. 192,396, filed February 24, 1938.

Heat treating operations for articles of various metals are oftencarried out by immersing the metal articles to be treated in a fusedsalt bath. This method of heat treatment is advantageous because of therapid transfer of heat from the bath to the metal articles, thusproviding a rapid process Well adapted to large scale production. Heattreating fused salt baths which contain no carburizing agent, forexample alkali metal cyanide or other cyanogen compound, are known tohave a more or less decarburizing effect upon steel surfaces, whicheffect has been overcome heretofore by introducing a carburizingmaterial into the bath. By this method the carburi- Zation which occursmore or less compensates for the decarburizing effect of the bath. Thismethod of controlling the carburizing activity of baths containing acyanogen compound is not particularly reliable or satisfactory.Moreover, it is often desired to employ baths which will exert asubstantial and controllable carburizing action upon steel articlesimmersed therein. So far as I am aware, there has been no reallysatisfactory method available for practically controlling thecarburizing activity of fused salt baths which contain a cyanogencompound.

It is an object of the present invention to provide a new and improvedmethod for controlling the activity of fused salt case hardening bathswhich contain a carburizing material. A further object is to provide amethod of operating fused salt baths containing a cyanogen compoundwhereby the case hardening activity of the bath may be inhibited orcontrolled as desired. Still further objects will be apparent from theensuing description of the invention.

The above objects are accomplished in accordance with my invention byheating metal articles in a fused salt bath containing a cyanogencompound, while maintaining said bath in an atmosphere of a gascontaining a predetermined amount of an oxidizing constituent. I havediscovered that a fused salt bath which contains a cyanogen compound,such as sodium cyanide, may be rendered substantially inert as regardscarburizing or decarburiz'ing activity, by maintaining the bath in anatmosphere of an oxygen-free, non-oxidizing gas, as described andclaimed in my copending application Serial No. 192,396, filed February24, 1938. I have also discovered that the case hardening activity ofsuch a bath increases as the amount of oxidizing constituent in thegaseous atmosphere maintained over the bath is increased, and that byvarying the amount of oxidizing constituent in the gaseous atmosphere,the case hardening activity of the bath may be varied accordingly.

The fused case hardening baths to be used in practicing the presentinvention of course must contain a carburizing material such as analkali metal cyanide. Alkaline earth metal cyanides and cyanamides, aswell as other cyanogen compounds, such as polymers of hydrocyanic acid,cyanamide, dicyandiamide and its polymers, may be used as carburizingbath ingredients. The amount of carburizing material present in the bathmay be varied over a wide range. Thus, the bath may contain 0.5'to 40%by weight of sodium cyanide, or if desired, a bath of substantially puresodium cyanide may be employed with good results.

The case hardening baths for use in practicing my invention preferablycontain as the chief bath constituents a cyanogen compound and an alkalimetal salt, an alkaline earth metal salt, or mixtures thereof. Inparticular, I prefer to use baths containing sodium cyanide togetherwith alkali metal and alkaline earth metal halides and carbonates, ormxtures thereof. The methods of mixing and fusing the salt ingredientsof the bath, as well as the proportions used to obtain baths havingmelting points adapted for use at the desired temperature of operation,will be known to those experienced in the preparation and use of fusedsalt baths for heat treating metals and need not be discussed here.

Specific examples of preferred fused salt baths for practicin myinvention are:

Bath NO. 1

Sodium cyanide: 15-30% by weight Remainder, sodium carbonate.

Bath N0. 2

Sodium cyanide, 220% by weight Remainder, equal parts of sodiumcarbonate and potassium carbonate.

Bath N0. 3

Sodium cyanide: 220% by weight Remainder, the mixture:

Per cent by weight Barium chloride 50 Potassium chloride 30 Sodiumchloride 20 Bath N0. 4

bath during operation. If an entirely inert or non-oxidizing atmosphereis employed, the bath will be substantially neutral, i. e., it willexert no substantial carburizing or decarburizing activity.Illustrations of non-oxidizng gases which may be employed to render thebath inert are hydrogen, nitrogen, gaseous hydrocarbons; the rare gasessuch as argon, krypton, and helium; and ammonia. When it is desired thatthe bath have a definite carburizing action, a suitable amount of anoxidizing substance is added to such an inert atmosphere. When there ispresent in the gaseous atmosphere over the bath a relatively smallamount of an oxidizing, constituent, a definite but small carburizingactivity will be exerted by the bath; the carburizing activity of thebath may be increased by increasing the amount of the oxidizing materialpresent in the gaseous atmosphere.

I prefer to employ oxygen as the oxidizing material which is added tothe gaseous atmosphere above the fused salt bath. However, otheroxidizing substances may be employed in accordance with my invention inplace of oxygen. For the purpose of the present invention, gases such ascarbon dioxide and water vapor are to be considered as oxidizing gasessince their action upon the activity of a salt bath when they are addedto the atmosphere above'such a bath is, generally speaking, the same asthat produced when oxygen is added.

Care should be exercised in choosing the gases to be used in making upthe gaseous atmosphere having a predetermined amount of an oxidizingconstituent. Thus, mixtures of oxygen with hydrogen, carbon monoxide orammonia should be avoided because of the explosive hazards involved. Iprefer to employ mixtures of nitrogen with suitable amounts of oxygen asgaseous coverings for the fused salt baths when operated in accordancewith the present invention.

Methods for maintaining an atmosphere having a predetermined oxidizingvalue over the case hardening bath will be obvious to those who arefamiliar with the art of heat treating metals and need not be describedin detail here. It will be sufficient to state that the carburizing bathmay be provided with a hood or mufile which is filled with the gaseousatmosphere containing the desired amount of an oxidizing substance. Aconvenient way of maintaning an atmosphere of suitable composition inthe muflie is to pass continuously a mixture containing suitableproportions of an inert gas and an oxidizing gas into the aperture ofthe muflie through 'which the articles to be treated are inserted orremoved from the bath. The gas mixture should be passed into the hoodedspace at such a rate that it streams out of the aperture (or 60 similaropenings in the muflie) through which work pieces are inserted orremoved, thereby preventing entrance into the hooded space of amounts ofair such as would be sufiicient'to change substantially the compositionof the gaseous atmosphere over the bath. If desired, suitable devicesmay be employed for passing the metal work into and out of the fusedsalt bath so that the treatment may be carried out in a continuousmanner. Thus when the case hardening of steel strip is involved, thesteel strip may be passed continuously through a fused salt bath, bymethods which are well known in the art, so as to insure that a givenportion of the strip remains immersed in the baththe required length oftime to efiect the desired case hardening. During operation, thecarburizing activity of the bath is controlled as desired by maintainingover the bath a gaseous atmosphere containing a predetermined suitableproportion of oxygen or other suitable oxidizing gas.

In general, my invention may be practiced to effectively control thecase hardening activity of case hardening baths in which cyanide orother cyanogen compound is used as the case hardening ingredient. Byvarying the proportion of oxidizing gas in the atmosphere above the casehardening bath, the case hardening activity of the bath may be variedaccordingly. For example, a cyanide case hardening bath may be operatedunder an atmosphere of nitrogen or other nonoxidizing gas and a suitableproportion of oxygen or other oxidizing gas may be mixed with thenitrogen. To decrease the activity of the bath, the atmosphere above thebath may for example consist of a nitrogen-oxygen mixture containing anoxygen concentration below that of the air, e. g., 1-5% by volume ofoxygen. To increase the activity of the bath, the covering atmospheremay, for example, consist of air to which oxygen has been added. Thismethod of control is generally applicable to case hardening bathscontaining cyanogen compounds and is not restricted to baths activatedwith cyanide.

My invention is not restricted to any particular temperature of heattreatment, but is generally applicable to the various heat treatingtemperatures commonly utilized for fused salt bath metal heat treatingoperations, e. g., 450-.

I claim:

1. A method for controlling the activity of a fused case hardening bath.which contains a cyanogen compound comprising maintaining said bathunder an atmosphere of a gas containing a predetermined amount of anoxidizing gas.

2. A method for controlling the activity of a fused case hardening bathwhich contains an alkali metal cyanide comprising maintaining said bathunder an atmosphere of a gas containing a predetermined amount ofoxygen.

3. A method for con-trolling the activity of a fused case hardening bathwhich contains an alkali metal cyanide comprising maintaining said bathunder an atmosphere of a gas mixture composed of a non-oxidizing gas andless than 20% by volume of oxygen.

4. A method for controlling the activity of a fused case hardening bathwhich contains an alkali metal cyanide comprising maintaining said bathunder an atmosphere of a gas mixture composed of a non-oxidizing gas and1-5% by volume of oxygen.

DONALD A. HOLT.

